Prestressed concrete railroad ties are known. These ties include a standard trapezoidal section usually having a broad, lower, and ground facing section of the ties reinforced with pre-stressed tensile elements, cables such as wire or strand. These wires or strands exert a compression force on the concrete of the tie, especially in the lower and ground facing portions of the tie. The ties at an upper and track supporting surface are manufactured with track fastening hardware integrally formed to the tie. Once the tie is placed, the track fastening hardware enables relative rapid rail placement and holds the rail precisely in place during the life of the tie.
When the ties are subjected to service loading--such as where a train passes over the ties--the pre-stressed concrete remains in compression. In what would otherwise be portions of a conventional wooden tie under tension become portions of the pre-stressed concrete tie under reduced--but not eliminated --compression. There results a concrete tie having superior wear characteristics over its wooden counterpart.
Such ties are manufactured in forms which in most cases define the tie dimensions. In all known cases, forms locate with precision the track fastening hardware. Consequently, a review of the manufacture of concrete railroad ties utilizing forms can be instructive.
In a conventional tie forming process, paired deadmen which resist the pre-tensioning force from the cables are located at opposite ends of a casting bed. Since the tie are cast to a mold, the bottom of which is the casting soffit, and are required to be released from the mold, casting occurs with the broad ground facing portion of the tie upwardly exposed and the track supporting upper portion of the tie with its track fastening hardware facing down. This requires the prestressing tie elements, cables, be elevated a distance from the casting soffit which is a further distance from the base of the casting bed, generally the ground.
Supporting elevated cables under high tension from deadmen is not trivial. The steel elements of the ties are high-strength steel usually stressed to 75% of their ultimate strength. Where the steel elements of the ties are supported in the order of over 6" from the ground, the deadmen at either end of the casting soffit must be designed to resist considerable torque relative to the casting bed. This being the case, it is common to firmly anchor pretensioning deadmen in buried and permanent foundations especially constructed to resist torque.
The forms are placed and distributed longitudinally along the casting bed with the pre-tensioning wires passing through the forms at the respective ends of the ties. The forms define in their lower surface, receptacles for the placement of the track fastening hardware. In one common process, form ends are defined by end gate bars. Once the forms are in place, concrete is poured, usually in conjunction with a vibratory force applied to the molds for consolidating the concrete.
When sufficient solidification has occurred--but before complete curing occurs, the gate end bars are removed leaving defined gaps at the tie ends. Once curing is complete, concrete sawing of the tie and tensile elements occurs at the interval defined by the now removed gate end bars. The discrete ties are then collected and shipped.
Having recited a process representative of the prior art, some of the disadvantages of conventional pre-stressed concrete railroad tie construction can be set forth.
First, and because the deadmen at either end of the casting soffit must resist considerable torque, facilities that manufacture concrete railroad ties are generally not portable; most facilities constitute permanent installations with deadmen having elaborate underground foundations which can never be conveniently moved. The concrete ties themselves are not easily shipped; commonly each conventional length tie weighs in the order of 750 pounds.
Second, the forms which mold the ties are expensive. Compounding this problem, changes in either the kind or location of rail fastening hardware requires replacement of the forms. Since such forms are custom made from steel stock, such replacement is expensive. Further, it is commonly required to discretely identify ties--especially as to production "batch". The forms must be modified to enable this identification. Unfortunately, the placement and manner of tie identification most always varies with each tie customer.
Third, where tie lengths change, form lengths likewise must change. When it is remembered that so-called switch ties come in many differing lengths, many forms are required for a single switch installation.
Fourth, combining the capital cost of deadmen and forms with other required accessories, plants for the production of concrete railroad ties are extremely expensive. Construction of a conventional concrete tie fabrication plant has an extremely high capital cost.
Fifth, the forms must be cleaned and maintained. Cleaning is required between each casting cycle. Form maintenance is required as forms age with use. For example, it is well known that forms, especially in the vicinity of the gate end bars, have wear induced gaps with repeating use. The gaps become points of grout leakage. Grout leakage leads to inconsistent strengths in the cast tie product, especially where such leakage occurs adjacent the tensile members of the tie.
Sixth, when the ties are in use, rail seat abrasion can occur. Specifically, a vulnerable point of the tie is adjacent to the tie fastening hardware where relative abrading movement of the rail relative to the tie and track fastening hardware can occur with the dynamic loading applied by passing rail wheels and their supported loads. Since ties are commonly constructed of a single consistent grade of concrete, abrasion at the tie support surface adjacent the rail fastening hardware is a common occurrence.
Attempts have been made to simplify concrete railroad tie construction utilizing slip forms. In Stinton et al. U.S. Pat. No. 4,253,817 entitled CONCRETE RAILROAD TIE CASTING AND HANDLING SYSTEM, molds defining the track support surface and holding the rail fastening hardware are placed in a casting bed and slip forming occurs over the molds. The tie is slip formed in an inverted disposition relative to the molds defining the track supporting surface and holding the hardware.
It is known to use slip forms--at least partially--for the construction of pre-stressed piles or slabs.